Adjustment Brush saves random settings as default. Sometimes (rarely) it updates them. And there is no way to reset default settings to all zeros. THIS IS VERY INCONVENIENT. Please fix the issue in the next update.
The default settings it selects are the last settings you selected BEFORE you adding a brush stroke. To change the defaults, select one of the local adjustment tools, make sure the Options panel is set to New rather than Edit mode, and adjust the sliders to your new chosen defaults. Lightroom remembers these settings and uses them for any new Graduated Filters, Radial Filters or Adjustment Brush strokes, until you next repeat the process.